A balanced alternating-photocurrent fluorophotometer

Abstract

An attempt is described to make a fluorophotometer to measure ratios of low fluorescence intensities from 1 to 1000 with an accuracy of 0.1%. An instability was found that limits the accuracy to about 5%. The photometer used two electron-multiplier photocells in a balanced circuit to eliminate the effects of fluctuations of the intensity of the light source. The construction of a 1000v regulated power supply for the electron multipliers is described. The light source is a mercury discharge lamp which causes alternating photocurrents; these are balanced directly without rectification, out-of-balance signal being indicated with a sensitive null-detector. The instability almost certainly arises from noise fluctuations in the electron multipliers and it is suggested that it would be eliminated by narrowing the band-width of the amplifier.